It is known that different ions (e.g. ions having different mass to charge values) may be optimally transmitted through or produced in a particular device under different operating conditions. It is also known to therefore scan or vary instrument parameters during the course of a mass spectrometry experiment so as to ensure adequate transmission or production of a range of ions of interest. This is particularly common in discovery-type experiments where the components being analysed are not necessarily known a priori.
One example of an instrument parameter that might be scanned in this way would be the collision energy within a fragmentation cell. By ramping collision energy over the course of an experiment a range of different ions can be efficiently fragmented that may not be achievable using any single collision energy.
Various mass spectrometry experiments are known where the collision energy is scanned or ramped to provide broader ion fragmentation coverage. Reference is made, for example, to WO 03/094197 (MDS). A development of this idea is described in WO 2011/091023 (WATERS TECHNOLOGIES CORPORATION) where a collision cell is repeatedly switched between high and low fragmentation modes approximately every second, with the collision energy in the high fragmentation mode also being varied as a function of time. The output of the collision cell is then passed to a mass analyser. A further experiment involving scanning collision energies is described in WO 2012/175978 (MICROMASS) where spectra are repeatedly recorded as the collision energy is scanned from low to high and back again from high to low in order to reduce settling times and avoid spectral skewing.
Other approaches to probing multiple collision energies are described in US 2014/0183347 (THERMO) and WO 2010/120496 (THERMO) wherein ions are passed to an intermediate ion storage unit as a series of discrete ‘fills’. Each fill may correspond to a different energy or method of collision activation of a selected precursor ion and the final ion population may thus correspond to an entire range of collision energies. However, all of the ions in each fill experience the same conditions and it will be appreciated, therefore, that in these experiments the changing the collision energy does not itself introduce any temporal modulation to different components of an ion beam.
Another example of an instrument parameter that might be scanned would be an RF potential applied to an ion guide being varied to transmit a different range of mass to charge values.
It is desired to provide an improved method of mass spectrometry.